치환적분

$\displaystyle \frac{\pi}{3} - x = t$로 놓으면, $\displaystyle - \frac{dx}{dt} = 1$이고 $x = 0$일 때 $\displaystyle t = \frac{\pi}{3}$, $\displaystyle x = \frac{\pi}{3}$일 때 $t = 0$이므로 $\displaystyle \int _{0} ^{\frac{\pi}{3}} {} \cos \left( \frac{\pi}{3} - x \right) dx$$\displaystyle = - \int _{\frac{\pi}{3}} ^{0} {} \cos t dt = \int _{0} ^{\frac{\pi}{3}} {} \cos t dt$ $\displaystyle = \left[ \begin{array}{l} \begin{matrix} \\ \end{matrix} \sin t \end{array} \right] _{0} ^{\frac{\pi}{3}} = \frac{\sqrt{3}}{2}$